The major difference is how each earns.
Debentures earn fixed interest while shares earn dividends.
If this comes as an exam question it is best to create a table and list the differences in contrast.
Other differences are that:
|Shareholders own the business
||Debenture holders are creditors|
|Share dividends are paid when a profit is made
||Debentures are paid in the presence or absence of profit generation|
|Shares have a risk on investment
||Debentures have no risk|
Using such a table and expanding on points where necessary will gain you full marks.